The state of the art showcases a wide variety of processes, technologies, and systems with whose help different surfaces, in particular connecting surfaces, can be coated with liquids, in particular mixtures of viscous polymers with solvents.
Adhesives represent an important group of liquids. Adhesives are used to an increasing extent in the semiconductor industry in order to connect substrates to one another. In this case, a distinction is made between the temporary adhesives and the permanent adhesives.
A temporary adhesive is in most cases a thermoplast, which softens above a characteristic temperature, the so-called glass transition temperature. Substrates that were glued with the help of a thermoplast can be separated from one another again by heating the thermoplast above the glass transition temperature. Thermoplasts can be thermally cycled; therefore, they can be theoretically heated several times and cooled again, without them losing their physical and/or chemical properties. The reason lies in the fact that in contrast to duromers, thermoplasts do not cross-link. As a result, the polymer chains can be moved toward one another at correspondingly high temperatures, and the temporary adhesive remains moldable.
Permanent adhesives are polymers whose polymer chains permanently cross-link to one another under chemical, thermal or electromagnetic action. The cross-linking process is irreversible. Such polymers are routinely used to connect substrates to one another, in particular substrates that are comprised of different materials.
The connection of two substrates can be performed by a wide variety of methods. Thus, for example, substrates with metal surfaces are connected to one another via a metal diffusion bond. The connection between silicon and/or silicon oxide surfaces is made by a so-called direct or fusion bond. Glass substrates can be connected to one another in particular also by anodic bonding if an ion transport that is required for this purpose is possible.
The above-mentioned connecting methods have the decisive drawback that they are limited to specific surface properties of the substrates. Thus, in most cases, only metal to metal surfaces can be connected by means of a metal diffusion bond. In addition, the direct or fusion bond technology can be used only between silicon and/or silicon oxide surfaces, or at least surfaces that have similar properties. For example, the bonding between a metal surface and a silicon surface in general is not possible by means of a diffusion bond or a direct bond. Therefore, such connections are mostly produced by means of permanent adhesives.
In principle, it is desired to minimize the thickness t of the permanent adhesive layer. On the one hand, the thickness t is to be large enough to connect the two substrates to one another securely and permanently, and preferably rough spots of the surface are to be compensated for. On the other hand, the thickness t should be as small as possible, however, in order not to impair the properties (for example, transparency, hardness, strength, rigidity, thermal conductivity) of the products that are produced.
The most important technique for applying adhesives is the centrifugal coating or centrifugal enameling. In the case of the centrifugal enameling, an amount of permanent adhesive is applied (i.e., dispensed) centrically on a substrate. In this case, the substrate is fixed to a specimen holder. After the application of the permanent adhesive, the specimen holder is set to rotating rapidly. Because of the centrifugal force, the permanent adhesive is distributed relatively uniformly over the substrate. The thickness t of the permanent adhesive can be set by the amount of deposition, the rotational speed, and the rotational acceleration.
Another coating technique is the spray coating or spray enameling. This is a technique in which a nozzle sprays a liquid that is to be applied while said nozzle performs a movement relative to a substrate. In the case of the relative movement between nozzle and substrates, this can be a simple rotation, a translation, or more complicated movements.
One of the greatest challenges in the connection of two substrates, which in most cases also still comprise different materials, is the production of a defect-free boundary layer. In the case of the direct connection of two substrates, in particular silicon-silicon or silicon oxide-silicon oxide, the focus is placed on the avoidance of inclusions and cavities. During direct bonding, the connecting surfaces of the two substrates are preferably centrically brought into contact with one another. Whereby a bonding wave proceeding from the center propagates radially outward. On the micro- and/or nano level, a distortion of one of the two connecting surfaces can occur. The distortion results in a bulging of at least one of the two connecting surfaces that remains permanently in the interface.
In contrast to the direct bonding method, the inclusion of gases, in particular the formation of bubbles, is the greatest problem in the case of the pet anent-adhesive-bonding method. The more highly viscous a liquid is, the more difficult it is for the gas inclusions to be able to escape from the liquid. An amount of gas, once trapped, can virtually no longer escape and remains in the interface.
Because of the coating methods used in the state of the art, the following problems arise.
The centrifugal enameling namely produces a relatively homogeneous and bubble-free layer on a substrate. However, the second substrate must be brought into contact with this already present permanent adhesive layer. Should the permanent adhesive layer have a high level of unevenness (roughness), inclusion of gas bubbles often results. In addition, all types of liquids, which are applied via centrifugal coating, form an edge bead at the edge of the substrates. Therefore, in the region of the edge bead, the permanent adhesive has a greater thickness than in the center. In this connection, gas inclusions often result. Most liquids applied by centrifugal enameling have such a high viscosity that they have to be diluted with a solvent. This solvent must be removed by a heat treatment process after the centrifugal enameling. Because of the outgassing of the solvent, the surface roughness increases, which in turn can lead to an inferior bonding result.
Even in the case of the spray coating, gas inclusions occur again and again, for example because of the roughness of the surfaces. This roughness is a direct result of the individual, agglomerating droplets on the surface of the substrate.